This invention relates to circuits for starting and operating gaseous discharge devices, both low and high pressure, such as, for example, fluorescent lamps and high pressure mercury vapor lamps. More particularly, it relates to an improved ballast apparatus especially adapted to provide improved operation of such gaseous discharge devices.
It is well known that the operation of a load which has a negative resistance characteristic, such as a gas discharge lamp, usually requires a series connected stabilizing impedance or ballast. The principal functions of the ballast are to provide the requisite operating and starting voltages for the electric discharge lamp and to regulate or limit the lamp operating current. Both passive and active types of ballast have been used to start and operate electric discharge devices such as fluorescent lamps or mercury vapor lamps. In the passive type of ballast a reactive impedance element is usually employed to provide the current regulating action necessitated because of the negative resistance characteristic of the electric discharge device. The reactive elements used for operating electric discharge devices with an alternating current source may be a simple inductor, or a high-leakage-reactance transformer in the case where a step-up voltage is necessary to reliably start or operate the gas discharge device. Capacitors are also used at times in conjunction with inductive elements to provide the current limiting ballast function for a discharge device. The types of inductive ballasts in common use are cumbersome and expensive because they generally require comparatively large iron cores and heavy windings in order to operate on the customary 60 Hertz AC power line. Similarly, the capacitors used are generally of the oil-filled type which use PCB type insulating oil as the dielectric fluid and are therefore a potential health hazard due to the toxic nature of the PCB oil.
In order to limit the size and expense of ballasts, a great variety of starting devices have been proposed, some of which are relatively complicated and actually add to the total cost of the system even while reducing the size and expense of the ballast element itself. One prior art solution entirely eliminates the ballast by operating the discharge lamp at a relatively high frequency, i.e. high relative to the 60 Hz AC supply voltage, but this solution has its own problems, for example, high frequency interference signals are generated which interfere with other electronic apparatus in the vicinity. There are also problems of reliability and power factor in making the conversion to high frequency.
Another solution, advanced in U.S. Pat. No. 3,383,554, adds a saturable reactor in series with the gas discharge tube to provide a current responsive variable impedance which maintains the tube discharge and provides a dimming function. It has been further proposed, in U.S. Pat. No. 3,496,412, to electrically interpose a rectangular wave generator between the source of AC power and the ballast circuit for the discharge lamp, thereby to prevent deionization of the gas when the supply voltage reverses polarity.
In general, it is an object of the invention to provide an improved ballast circuit for starting and operating a negative resistance device such as an electric discharge lamp.
Another object of the invention is to improve the operating characteristics of a conventional inductive-capacitive type ballast apparatus.
A further object of the invention is to increase the efficiency of a ballast system used with gas discharge lamps.
In the case where a transformer or inductor ballast is used, it is known to connect a fixed value capacitor in series therewith. The purpose of this capacitor is to provide a leading lamp current and to regulate or control the lamp current despite variations in the AC supply voltage. A further purpose of the capacitor is to provide power factor correction through saturation control of the magnetic circuit.
One figure of merit of a ballast circuit is the "crest factor" of the lamp current waveform, which is defined as the ratio of the peak lamp current to the rms value of lamp current. A typical ballast operating crest factor is 1.60. It is generally agreed that the crest factor should be kept below 1.7 in order to avoid undue shortening of the lamp life. A true sinusoid lamp current would have a crest factor of 1.41, whereas a perfect squarewave would be 1.0, the optimum value to produce maximum lamp efficiency. In general, the lamp current waveforms obtained with most ballast devices exhibit a distorted sinusoidal waveshape which reduces the lamp efficiency as well as its useful life.
It is therefore a further object of the invention to provide an improved ballast circuit with a crest factor that more nearly approaches the ideal square wave characteristic.
Another object of the invention is to provide a novel apparatus for improving the efficiency of an inductive type ballast by reducing the crest factor of the discharge lamp current waveform.
It is a further object of the invention to provide an improved apparatus for compensating for the distortion of the lamp current waveform caused by saturation of the iron core of a ballast coil or transformer.